The invention relates to the field of monitoring temperature for a diffusion furnace used in the processing of semiconductor material.
FIG. 1 shows a simplified view of a prior art diffusion furnace system used for processing semiconductor materials. The furnace 100 includes a cylindrical container 102 having a source area 104, a center area 106 and a handle area 108. Typically silicon wafers are positioned in the cylinder and gases are injected into the cylinder at the source end 110 of the container. A heating element 112, which consists of a coil wrapped around the outside of the container 102 is used to heat the container 102 and its contents. A heating apparatus 114 is used to heat the coil 112. A controller 116 is used to monitor the temperature of the container 102 and control the heating apparatus 114 driving the coils 112. For the processing of the semiconductor material inside the container 102 to be effective the temperature must be precisely controlled. Thus, the temperature of the container 102 must be accurately monitored.
The prior art system has three temperature monitoring zones 118, 124 and 130 which are disposed in the source area 104, the center area 106, and the handle area 108, respectively. The source zone 118 includes three thermocouples positioned in the source area 104 of the container. Two of the thermocouples 120 are referred to as spike thermocouples. As shown the spike thermocouples 120 are located outside of the container 102 on opposite sides of the container 102 and positioned between adjacent windings of the heating element coil 112. In practice however the spike thermocouples may be positioned adjacent to each other. The third thermocouple 122 in the source zone 118 is located inside the container 102, and is referred to as the profile thermocouple. In a similar manner the center monitoring zone 124 has two spike thermocouples 126 positioned on the outside surface of the container 102, between adjacent windings of the coil 112, and profile thermocouple 128 positioned inside the container 102. In a similar manner, a handle monitoring zone 130 is created in the handle area 108. The handle zone 130 includes two spike thermocouples 132 and a profile thermocouple 134.
A thermocouple is a heat sensing device which consists of dissimilar metals which are joined together. Other heat sensing devices which operate in manner similar to a thermocouple could also be used. The junction between the metals of the thermocouple is such that when it is exposed to heat it will generate a voltage. The more heat the thermocouple is exposed to the higher the resulting voltage. Conversely, as the temperature is lowered the voltage will decrease. In the prior system 100, the thermocouples are coupled to a controller 116 which operates to sense the voltage for each of the thermocouples. If the voltage of the thermocouples falls below a certain threshold then the power driving coil 112 will be increased by the heating apparatus 114, in response to signals from the controller 116. If the voltage of the thermocouples exceeds a certain threshold then the controller 116 will cause the heating apparatus to decrease the power driving the coil 112, thereby decreasing the generated heat sensed by the thermocouples.
In the prior art system 100, a problem can arise when one of the thermocouples fails. Typical failures for a thermocouple are manifested in one of two ways. A thermocouple may fail to generate a voltage in response to heat. In this case the thermocouple essentially shorts out. In this situation, even if the thermocouple is exposed to a very high temperature, it will fail to generate a voltage.
A thermocouple can also fail by going to a state where it becomes an open circuit. In this situation even when the thermocouple is exposed to very little heat it will appear to be generating a high voltage relative to a thermocouple that has not failed due to an open condition.
If the thermocouple has a short failure then the controller 116, detecting a very low voltage at the thermocouple, will process this detection as if the thermocouple were generating too little voltage as a result of the temperature being to low. Accordingly, the controller 116 will cause the heating apparatus 114 to drive the coil 112 to generate higher temperatures. Thus, increasing the temperature of the container 102. When there is a short failure, the controller 116 will frequently cause the heating apparatus 114 to drive the coil 112 to increase temperature above desired levels, which can result in a failed process.
If one of the thermocouples has an open failure, the controller 116 will sense what appears to be a very high voltage, which would lead to less power being used to drive the coil 112. In prior systems the thermocouple with the lower voltage was deemed to be the one on which the controller would base the control of the heating apparatus. As a result it was not uncommon to see a processes fail as a result of being overheated, where one of the thermocouples had to short failure. Further, these prior systems did not provide any easy way for a user to detect when a thermocouple failed due to a short condition.
In these prior systems, when a thermocouple failed because it was in an open condition, the operation of the furnace would likely continue successfully for a time, until a second thermocouple failed, at which point, the system operation could fail. If the second thermocouple failed as result of being open, then the controller 116 would allow the temperature to drop to low. If the second thermocouple failed as short then the system operation would fail as result to the temperature being driven to high. What is needed is a system which provides a simple and intuitive user interface which alerts a user if a thermocouple fails, and which makes optimum use of the thermocouples to increase the probability that the processing of the semiconductor material in the diffusion chamber will be successful.
The heat sensing device manager system and method provided herein, offer the advantage of detecting whether a heat sensing device has failed due to an open condition or a short condition. In particular when a device is determined to have failed due to a short condition, the heat sensing manager system and method will cause the controller to not use the signal from the shorted heat sensing device as a basis for controlling the heating apparatus of the system. In addition, an embodiment of the system can provide a very simple LED user interface that conveys information about the operation of the heat sensing devices of the system. In particular the user interface can indicate which, if any, of the plurality of heat sensing devices have failed, and whether a failure of the heat sensing device is due to an open condition or a short condition.